An automobile or car is typically fueled at a service station by a gasoline dispensing apparatus which transfers a volume of liquid gasoline (V.sub.liquid) from a storage tank into the car's fuel tank. When the volume of liquid gasoline (V.sub.liquid) is dispensed into the car's fuel tank, it displaces the same volume of fuel/air vapor within the car's fuel tank (V.sub.car). A gasoline recovery system is usually provided to prevent the excessive release of this fuel/air vapor into the atmosphere at the vapor. This recovery system transfers fuel/air vapor released from the car's fuel tank back to the storage tank. The storage tank is usually situated underground and includes a vent for releasing vapor and introduced air to equalize the pressure of the storage tank.
In a typical gasoline dispensing apparatus, a fuel hose extends from the storage tank to a nozzle that may be selectively triggered to dispense the liquid gasoline into the car's fuel tank. The same fuel hose is also commonly used to transport the displaced air/fuel vapor back to the storage tank. A pump is provided to transport the liquid gasoline through the fuel hose and a liquid flow meter is provided to measure the volume of the liquid gasoline (V.sub.liquid) delivered to the car's fuel tank. A suction pump is provided to transport the displaced air/fuel vapor through the fuel hose and the flow rate of this suction pump is controlled by the gasoline recovery system to achieve the desired flow rate. For example, if a variable speed suction pump is being used, the speed of the pump may be modulated. Alternatively, if a constant speed suction pump is being used, the position of appropriately placed dampers or valves may be changed.
The escape or release of fuel vapor into the atmosphere is referred to as "emission" and is usually expressed in terms of the percentage volume of pure gasoline vapor relative to the volume of dispensed liquid gasoline (V.sub.liquid). "Nozzle emission" occurs if the volume of air/fuel vapor (V.sub.car) transported back to the storage tank is less than the volume of displaced air/fuel vapor (V.sub.liquid). "Vent emission" occurs if the storage tank becomes over pressurized and its vent is opened to release excess vapor. "Total emission" refers to the sum of nozzle emission and the vent emission.
As was indicated above, the liquid gasoline and the displaced air/fuel vapor are usually transported by the same fuel hose and these fluids are usually in a heat-exchanging relationship relative to each other as they counter flow through the fuel hose. For example, a fuel hose may include a central passageway through which the liquid gasoline is transported to the car's fuel tank and an outer annular passageway through which the displaced air/fuel vapor is transported to the storage tank. Assuming that the car's fuel tank is at the ambient temperature and the storage tank is situated underground, there will usually be a temperature differential between the car's fuel tank and the storage tank (T.sub.car.noteq.T.sub.storage) whereby a heat transfer will occur between the counter flowing fluids. Consequently, the volume of the air/fuel vapor withdrawn from the car's fuel tank will be different than the volume of the air/fuel vapor introduced into the storage tank (V.sub.car.noteq.V.sub.storage).
In winter conditions, the temperature of the car is less than the temperature of the storage tank (T.sub.car &lt;T.sub.storage) and the volume of the vapor expands as it transferred from the car's fuel tank to the storage tank (V.sub.car &lt;V.sub.storage). To eliminate nozzle emission, the recovery rate may be set so that the volume of vapor removed from the car's fuel tank (V.sub.car) is the same as the volume of liquid gasoline dispensed thereinto. (V.sub.car =V.sub.liquid.) However, this would result in volume of vapor delivered to the storage tank (V.sub.delivered) being greater than the volume of liquid gasoline removed from the storage tank (V.sub.liquid). (The volume of the liquid gasoline (V.sub.liquid) remains essentially unchanged since its thermal expansion coefficient is only about 0.0001/.degree. C.)
The percentage increase in the volume of the vapor as it is transported from the car to the storage tank (V.sub.delivered -V.sub.car) due to thermal expansion is equal to the absolute temperature ratio (t=T.sub.storage /T.sub.car &gt;1). To eliminate excess volume due to thermal expansion, the volume of vapor removed from the car's fuel tank (V.sub.car) may set to compensate for thermal expansion (V.sub.car =t V.sub.liquid). Since the volume of air/fuel vapor recovered (V.sub.car) is not less than the volume of air/fuel vapor displaced in the car's fuel tank (V.sub.liquid), nozzle emission is eliminated. However, since the volume of air/fuel vapor recovered (V.sub.car) is greater than the volume of air/fuel vapor displaced in the car's fuel tank (V.sub.liquid) excess air must be added to the recovery vapor.
When the excess air in the recovery vapor reaches the storage tank, it saturates with "fresh" gasoline vapor. This causes the just-delivered volume of air/fuel vapor to expand by a saturation factor of (1+.DELTA.p/p.sub.ambient) wherein .DELTA.p is the difference between the saturation pressure of the fuel vapor of the storage tank conditions (p.sub.sat (storage)) and the fuel vapor pressure at the car's fuel tank (p.sub.vapor (car)). Thus, the saturation of vapor in the storage tank results in excess vapor volume in the storage tank (V.sub.liquid t.DELTA.p/p.sub.ambient). This excess volume is released via the storage tank vent causing a vent emission equal to the released excess vapor volume times the relevant saturation factor (p.sub.sat (T.sub.storage)/p.sub.ambient)).
By way of specific example, suppose that the fuel vapor is pentane (C.sub.5 H.sub.12), the temperature of the car is 290.degree. K (T.sub.car) and the temperature of the storage tank is 300.degree. K (T.sub.storage). If volume of vapor removed from the car's fuel tank (V.sub.car) is set to account for thermal expansion (1.033 V.sub.liquid), there would be an approximately 34% percent (1.34 V.sub.liquid) volume excess in the tank due to saturation expansion. The excess vapor volume released through the storage tank's vent would cause a storage emission, in percentage of the volume of dispensed liquid gasoline (V.sub.liquid), of about 25% pentane. Thus, while nozzle emission is eliminated, storage emission is relatively high whereby total emission is also relatively high.
The present invention includes the appreciation that, during winter conditions, total emission may be minimized by taking into account the saturation expansion of the air/fuel vapor delivered to the storage tank. To this end, the present invention provides a gasoline vapor recovery system wherein, during winter conditions, the volume of the air/fuel vapor delivered to the storage tank (V.sub.delivered) is less than the volume of liquid gasoline (V.sub.liquid) but that will expand upon saturation in the storage tank to be approximately equal to the volume of the liquid gasoline (V.sub.liquid).
Preferably, the volume of vapor delivered to the storage tank (V.sub.delivered) is based on the fuel vapor saturation pressure at the temperature of the storage tank (p.sub.sat (T.sub.storage)). More preferably, the volume of vapor delivered to the storage tank (V.sub.delivered) is based on the equation: EQU V.sub.delivered =V.sub.liquid / (1+.DELTA.p/p.sub.ambient)
wherein .DELTA.p=p.sub.sat (storage)-p.sub.vapor (delivered); PA1 wherein p.sub.sat (storage)=saturation vapor pressure for the fuel at the temperature of the storage tank ; and PA1 wherein p.sub.vapor (delivered)=vapor pressure at the rate-controlling device.
The saturation vapor pressure (p.sub.sat (storage)) is preferably determined by measuring the temperature of the storage tank (T.sub.storage) and then using this value to determine the saturation vapor pressure. Specifically, a stored table of saturation pressures at different temperatures for the particular fuel being pumped allows a look-up of the saturation pressure (p.sub.sat) at the measured temperature of the storage tank (T.sub.storage). The vapor pressure at the rate-controlling device is preferably determined by the concentration (c.sub.delivered) of the vapor as delivered to the storage tank (such as by measuring its thermal conductivity) and then using this value to determine the saturation pressure.
Once within the storage tank, the volume of delivered vapor (V.sub.delivered) saturates to expand to a volume approximately equal to the volume of liquid gasoline removed (V.sub.liquid). In this manner, the storage tank remains at equilibrium pressure and vent emission is eliminated. It may be noted that, however, since .DELTA.p is greater than zero, the volume of vapor removed from the car's fuel tank (V.sub.car) is less than the volume of liquid gasoline dispensed therein (V.sub.liquid). This volume differential (V.sub.liquid -V.sub.car) will be released at the nozzle thereby accounting for a nozzle emission of this amount times a concentration factor (p.sub.sat (T.sub.car)/p.sub.ambient).
In context of the above specific fuel and temperature example, removing a volume of vapor from the car's fuel tank (V.sub.car) equal to 74.6% (100/1.34) of the dispensed liquid gasoline (V.sub.liquid) would lead to an exact post-saturation volume match at the storage tank whereby there would be no vent emissions. Since the volume of vapor removed from the car's fuel tank (V.sub.car) is less than the liquid gasoline dispensed therein (V.sub.liquid), nozzle emission does occur of about 12.7% pure pentane (in percentage to the volume of dispensed liquid gasoline (V.sub.liquid)) and thus there is a 12.7% total emission. Thus, while this method does not eliminate nozzle emission, it does substantially reduce total emission when compared to a method aimed at totally eliminating nozzle emission.
Accordingly, the present invention also includes the appreciation that total emission may be minimized in winter conditions if nozzle emissions are minimized instead of eliminated.
In summer conditions, the temperature of the car is greater than the temperature of the storage tank (T.sub.car &gt;T.sub.storage, t&lt;1) and the withdrawn vapor is cooled by the counter-flow and its volume contracts. (V.sub.car &lt;V.sub.storage). If the volume of vapor withdrawn from the car's fuel tank (V.sub.car) is set equal to the volume of the liquid gas dispensed therein (V.sub.liquid), nozzle emission will be eliminated. However, the volume of vapor delivered to the storage tank will be less than the volume of liquid fuel dispensed therefrom (V.sub.liquid &gt;V.sub.storage) thereby creating a vacuum within the storage tank. This vacuum is equalized by air being brought in through the storage tank's vent. The newly introduced air saturates with the vapor thereby expanding the vapor volume in the storage tank by a factor of (1-t) (1+p.sub.sat (T.sub.storage)/p.sub.ambient). The excess volume (V.sub.liquid (1-t)) p.sub.sat (T.sub.storage)/p.sub.ambient) is then released through the vent to equalize the pressure of the storage tank, a fraction p.sub.sat (T.sub.storage)/p.sub.ambient of which is pure fuel vapor.
By way of specific example, suppose again that the fuel vapor is conservatively assumed to be pentane (C.sub.5 H.sub.12), and that the temperature of the car (T.sub.car) is 300.degree. K and the temperature of the storage tank (T.sub.storage) is 290.degree. K. Withdrawing a volume of vapor at the nozzle (V.sub.car) equal to the volume of liquid gasoline dispensed into the car's fuel tank (V.sub.liquid), would result in a 52.3% expansion of the volume of air drawn in through the storage tank's vent and a 27.4% vent emission of pure pentane volume.
The present invention provides a gasoline vapor recovery system wherein, during summer conditions, the volume of vapor delivered to the storage tank (V.sub.delivered) is saturated and equal to the volume of liquid gasoline withdrawn from the storage tank (V.sub.liquid). This results in the volume of vapor withdrawn at the nozzle (V.sub.car) being greater than the volume of liquid gasoline dispensed into the car's fuel tank (V.sub.liquid) whereby nozzle emission is eliminated. The present invention includes the appreciation that, during summer conditions, any saturation of excess air withdrawn with the vapor will occur in the fuel hose, prior to the vapor being introduced into the storage tank. Accordingly, the volume of vapor introduced to the storage tank (V.sub.storage) is equal to the volume of liquid gasoline (V.sub.liquid) dispensed from the storage tank whereby vent emission is also eliminated.
Accordingly, the present invention provides a gasoline vapor recovery system wherein total emission is minimized by taking into account potential saturation expansion of a volume of air/fuel vapor after it is delivered to the storage tank. During winter conditions, nozzle emission may not be totally eliminated but total emission is minimized. During summer conditions, both nozzle emission and vent emission may be eliminated.
These and other features of the invention are fully described and particularly pointed out in the claims. The following description and drawings set forth in detail a certain illustrative embodiment of the invention, these embodiments being indicative of but one of the various ways in which the principles of the invention may be employed.